A commercially available optical disk at present such as a compact disk (CD) has a disk substrate thickness of 1.2 mm, and a pitch of 1.6 .mu.m of a track for recording information. For the purpose of playing back such a CD, an optical head provided with a semiconductor laser emitting a laser beam having a wavelength of 0.78 .mu.m and an objective lens having a numerical aperture (NA) of 0.45 is employed. Regenerating methods commonly used for processing an information signal derived by an optical head from a CD include waveform equalization based on a transversal equalizer or the like and a method based on digitization.
An optical disk having a recording density higher than that of a CD has recently been standardized to serve as an optical recording medium. The standardized specifications for a digital disk (DVD), one of the high-recording-density optical disks set forth a disk substrate thickness of 0.6 mm and a track pitch of 0.74 .mu.m, and for this DVD, use is now considered of an optical head which irradiates a laser beam having a wavelength of 0.65 .mu.m or 0.635 .mu.m through an objective lens having a numerical aperture of 0.6. There is proposed an optical recording apparatus capable of processing optical disks of a plurality of specifications including not only an optical disk having a high recording density such as a DVD, but also conventional CDs.
However, a write-once type CD (CD-R) used as commonly as a CD at present cannot be employed in an optical recording apparatus playing back a DVD by the use of a laser beam of a short wavelength for DVD and capable also of playing back a CD. The CD-R is used for production of a slight quantity of CD-ROMs, data backup or trial manufacture of CDs, and is a dye-based optical recording medium having a high wavelength selectivity near the wavelength (0.78 .mu.m) of a laser beam for a CD. In the proximity of the standard wavelength (0.65 .mu.m), the reflectivity is too low to be practicable. Specifications for wavelengths of a laser beam used for recording or regeneration on or from an optical disk are usually determined by taking account of the track pitch. For a CD, for example, the ratio of the light source wavelength (0.78 .mu.m) to the track pitch (1.6 .mu.m) is 0.49, and for a DVD, the ratio of the light source wavelength (0.65 .mu.m) to the track pitch (0.74 .mu.m) is 0.88. All standards use a laser beam having a wavelength far shorter than the track pitch. In an optical recording apparatus using a laser beam having a short wavelength in conformity to these standards, therefore, regeneration is made possible for DVD and a read-only CD by using a two-focus hologram so as to condense the laser beam onto a CD and DVD having different disk thicknesses, whereas compatibility with a CD-R cannot be ensured in this case. Consequently, it is still necessary to use a separate optical recording apparatus based on a laser beam having a longer wavelength for the purpose of accomplishing recording or regeneration of a CD-R.
The present invention has therefore an object to provide an optical head and an optical recording apparatus which permit recording or regeneration in a group of a CD of any specifications for a DVD, a read-only CD and a CD-R. It is also an object of the invention to provide an optical head and an optical recording apparatus capable of processing a currently commercially available CD-R as well as a DVD permitting high-density recording with a single unit of head or apparatus. Another object of the invention is to provide an optical head and an optical recording apparatus which ensure compatibility of a digital video disk (DVD) and a next-generation high-density optical disk with a currently commercially available CD and a conventional write-once CD (CD-R), achievement of a further higher density, and expansion of manufacturing margin.